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#include <stdio.h>
#include <stdlib.h>
#include <math.h>
 
/* write_bmp_header():
 *
 *   In:
 *      f: A file open for writing ('w') 
 *      (width, height): image dimensions
 *   
 *   Return:
 *      0 on success, -1 on failure
 *
 */
 
int write_bmp_header(FILE *f, int width, int height) {
 
  unsigned int row_size_in_bytes = width * 3 + 
      ((width * 3) % 4 == 0 ? 0 : (4 - (width * 3) % 4));
 
  // Define all fields in the bmp header
  char id[2] = "BM";
  unsigned int filesize = 54 + (int)(row_size_in_bytes * height * sizeof(char));
  short reserved[2] = {0,0};
  unsigned int offset = 54;
 
  unsigned int size = 40;
  unsigned short planes = 1;
  unsigned short bits = 24;
  unsigned int compression = 0;
  unsigned int image_size = width * height * 3 * sizeof(char);
  int x_res = 0;
  int y_res = 0;
  unsigned int ncolors = 0;
  unsigned int importantcolors = 0;
 
  // Write the bytes to the file, keeping track of the
  // number of written "objects"
  size_t ret = 0;
  ret += fwrite(id, sizeof(char), 2, f);
  ret += fwrite(&filesize, sizeof(int), 1, f);
  ret += fwrite(reserved, sizeof(short), 2, f);
  ret += fwrite(&offset, sizeof(int), 1, f);
  ret += fwrite(&size, sizeof(int), 1, f);
  ret += fwrite(&width, sizeof(int), 1, f);
  ret += fwrite(&height, sizeof(int), 1, f);
  ret += fwrite(&planes, sizeof(short), 1, f);
  ret += fwrite(&bits, sizeof(short), 1, f);
  ret += fwrite(&compression, sizeof(int), 1, f);
  ret += fwrite(&image_size, sizeof(int), 1, f);
  ret += fwrite(&x_res, sizeof(int), 1, f);
  ret += fwrite(&y_res, sizeof(int), 1, f);
  ret += fwrite(&ncolors, sizeof(int), 1, f);
  ret += fwrite(&importantcolors, sizeof(int), 1, f);
 
  // Success means that we wrote 17 "objects" successfully
  return (ret != 17);
}
 
 
/*
 * compute_julia_pixel(): compute RBG values of a pixel in a
 *                        particular Julia set image.
 *
 *  In:
 *      (x,y):            pixel coordinates
 *      (width, height):  image dimensions
 *      tint_bias:        a float to "tweak" the tint (1.0 is "no tint")
 *  Out:
 *      rgb: an already-allocated 3-byte array into which R, G, and B
 *           values are written.
 *
 *  Return:
 *      0 in success, -1 on failure
 *
 */
 
int compute_julia_pixel(int x, int y, int width, int height, float tint_bias, unsigned char *rgb) {
 
  // Check coordinates
  if ((x < 0) || (x >= width) || (y < 0) || (y >= height)) {
    fprintf(stderr,"Invalid (%d,%d) pixel coordinates in a %d x %d image\n", x, y, width, height);
    return -1;
  }
 
  // "Zoom in" to a pleasing view of the Julia set
  float X_MIN = -1.6, X_MAX = 1.6, Y_MIN = -0.9, Y_MAX = +0.9;
  float float_y = (Y_MAX - Y_MIN) * (float)y / height + Y_MIN ;
  float float_x = (X_MAX - X_MIN) * (float)x / width  + X_MIN ;
 
  // Point that defines the Julia set
  float julia_real = -.79;
  float julia_img = .15;
 
  // Maximum number of iteration
  int max_iter = 300;
 
  // Compute the complex series convergence
  float real=float_y, img=float_x;
  int num_iter = max_iter;
  while (( img * img + real * real < 2 * 2 ) && ( num_iter > 0 )) {
    float xtemp = img * img - real * real + julia_real;
    real = 2 * img * real + julia_img;
    img = xtemp;
    num_iter--;
  }
 
  // Paint pixel based on how many iterations were used, using some funky colors
  float color_bias = (float) num_iter / max_iter;
  rgb[0] = (num_iter == 0 ? 200 : - 500.0 * pow(tint_bias, 1.2) *  pow(color_bias, 1.6));
  rgb[1] = (num_iter == 0 ? 100 : -255.0 *  pow(color_bias, 0.3));
  rgb[2] = (num_iter == 0 ? 100 : 255 - 255.0 * pow(tint_bias, 1.2) * pow(color_bias, 3.0));
 
  return 0;
}
 
 
int main(int argc, char **argv){
    if(argc != 2){
        fprintf(stderr, "Usage: %s <N_integer_positive>\n", argv[0]);
        exit(EXIT_FAILURE);
    }
    int n = atoi(argv[1]);
    if(n % 2 == 1){
        fprintf(stderr, "n (%d) should be even!\n", n);
        exit(EXIT_FAILURE);
    }
 
    unsigned char *v = (unsigned char *) malloc(n * 2 * n * 3 * sizeof(unsigned char));
    int x,y ;
 
    for(x = 0; x < 2 * n; x++)
        for(y = 0; y < n; y++)
            if(compute_julia_pixel(x, y, 2 * n, n, 1.0, &v[x * n + y]) == -1)
                printf("error computing pixel (%d %d)\n", x, y);
 
 
 
    FILE *f = fopen("julia.bmp", "w");
 
    if(write_bmp_header(f, 2 * n, n) == -1)
        printf("error writing the header \n");
 
    for(x = 0; x < 2 * n; x++)
        for(y = 0; y < n; y++)
            fwrite(&v[x * n + y], sizeof(char), 3, f);
 
    fclose(f);
    free(v);
 
    return 0;
}